What defines a pure monopoly and creates barriers to entry?

"At the other end of the spectrum is pure monopoly, the market structure in which a single firm accounts for all industry sales of a particular good or service. The firm is the industry. This market structure is characterized by barriers to entry—factors that prevent new firms from competing equally with the existing firm. Often the barriers are technological or legal conditions. Polaroid, for example, held major patents on instant photography for years. When Kodak tried to market its own instant camera, Polaroid sued, claiming patent violations. Polaroid collected millions of dollars from Kodak. Another barrier may be one firm’s control of a natural resource. DeBeers Consolidated Mines Ltd., for example, controls most of the world’s supply of uncut diamonds. Public utilities, such as gas and water companies, are pure monopolies. Some monopolies are created by a government order that outlaws competition. The U.S. Postal Service is currently one such monopoly."